A grand jury has indicted three Texas men for illegally buying guns for others, including one used in the shooting death of a U.S. immigration agent in Mexico, the Justice Department said on Thursday.

The indictment returned on Wednesday accused Ranferi Osorio, his brother Otilio Osorio and Kelvin Morrison of conspiracy and buying firearms by making false statements. The brothers were also charged with owning a firearm with the serial number removed or destroyed.

Otilio Osorio bought a 7.62 caliber pistol on October 10, 2010, from a Texas dealer and ballistics testing indicated it was one of three firearms used in an attack on two U.S. Immigration and Customs Enforcement agents in Mexico in February, prosecutors said earlier this month.

One of the ICE agents, Jaime Zapata, was killed and the second agent, Victor Avila, was wounded. They were attacked while driving back to Mexico City in an armored sports utility vehicle on a major highway in broad daylight.

The indictment, filed in federal court in Dallas, Texas, did not include any charges related to that shooting. The three men, all from the Dallas area, were arrested on February 28 and remain in custody.

Weapons and drug trafficking has been an escalating problem along the U.S. border with Mexico. Authorities have been particularly concerned about people buying weapons in the United States for the purpose of selling them to drug cartels in Mexico, a practice known as straw gun purchases.

Source: Reporting by Jeremy Pelofsky, editing by John Whitesides for Reuters.